A transparent conductive thin film is a material widely used for devices such as image sensors, solar cells, various displays (PDP, LCD) and the like, which need to have light penetration and conductivity. Indium tin oxide (ITO) has been studied as a generally flexible transparent electrode for displaying, but ITO has disadvantages, in that it causes expansive processing costs since a vacuum process environment is basically required for manufacturing ITO thin film and its life span is shortened due to ITO thin film being broken off when displaying devices are bent or folded.
To solve the above-mentioned problems, a transparent electrode whose visible ray penetration is more than 80% and sheet resistance is less than 100 Ω/sq has been developed by minimizing light scattering and improving conductivity in the visible ray region through a process in which CNTs were dispersed in or on the surface of a coating layer at nano scale by a film of CNTs after chemically bonding it with polymers or coating purified CNTs or CNTs chemically bonded with polymers on a conductive polymer layer, and then metal nanoparticles such as gold and silver, etc., were mixed therein (KR 10-2005-001589). However, the transparent electrode has disadvantages in that it may alter inherent properties of CNTs and have polymer deformation when it is used at high temperature.
Moreover, a method for forming various wires on a plastic substrate with a general office inkjet printer using a conductive ink-jet ink having a specific resistance of 0.2 Ωcm, which is prepared by adding cut CNTs and gold nanoparticles to a solution containing aniline, a conductive polymer dissolved in isopropylalcohol (KR 10-2005-0080960), has been developed but the method has a disadvantage, in that penetration of wires and patterns printed on a substrate is poor.
Besides, although studies on using conductive polymers, which are organic materials, as a material for transparent electrodes are being conducted, most of the conductive polymers for transparent electrodes developed so far are not suitable to use for transparent electrodes since they absorb light in the visible ray region.
To solve the above mentioned problems, the present inventors have developed a method for manufacturing a transparent electrode using a high density CNT film having high purity (KR 10-2006-0032812). The above mentioned invention has an advantage in that electrical conductivity of the CNT film is improved while maintaining a visible ray penetration into the CNT film by forming metal nanoparticles on the surface of the high density CNT film formed on a substrate with covalent bonds or non-covalent bonds, by means of the electroless plating technique using an aquous solution of metal salts. However, there is a problem in that it is difficult to apply to a large-area film since the increased electrical conductivity is not uniform over the whole film.
Thus, there is an urgent need to develop a method for manufacturing a transparent electrode composed of CNT film having high visible ray penetration and excellent electrical conductivity, as well as a uniform increase in electrical conductivity over the whole CNT film.
Accordingly, the present inventors have made extensive efforts to manufacture a transparent electrode having high visible ray penetration and excellent electrical conductivity, and a uniform increase in electrical conductivity over the whole CNT film. As a result, the present inventors have found that, when a CNT film was formed on a desired substrate using a dispersed solution of CNTs and a transparent electrode is then produced by reducing metal nanoparticles on the surface of the CNT film, the produced transparent electrode showed a uniform increase in electrical conductivity, thereby completing the present invention.